1. Field of the Invention
The present invention relates to an exposure control system for a fixed focus lens camera of the type that forms a subject image through a scanning aperture, in general, and to such a system that includes a simplified arrangement for determining if the subject is within or outside of the lens's normal depth of field for exposure control purposes, in particular.
2. Description of the Prior Art
It is well known that the sharpness of an image formed by a lens at an image plane is primarily dependent upon the exactness of focus of the image at the image plane. When a lens is focused to produce a sharp image of a particular object at the image plane, other objects that are closer or further away do not appear equally sharp. The decline in sharpness is gradual and there is a spacial zone extending in front of and behind the focused subject where the image misfocus is too small to be noticeable and therefore can be accepted as sharp. This zone is commonly referred to as the depth of field of the lens.
The initial photographic cameras only employed lenses of the fixed focus type. In order to form photographic images of acceptable sharpness with such lenses over the greatest possible range of subject distances, their optical characteristics had to be chosen such that the near distance of their depth of field could only extend to within approximately 4 to 5 feet of the camera for an f/14 lens, if images of distant objects (objects at infinity) were to have an acceptably sharp focus at the camera's film plane. An image of an object produced by this type of lens that is located closer to the camera than this near distance would appear blurred or noticeably out of focus.
The production of sharp images of distant as well as relatively close objects is not a problem with an adjustable focus lens. With such a lens, the focus distance and its associated depth of field can be adjusted to produce acceptably sharp images of objects located at virtually any object distance. While an adjustable focus lens has many advantages, including the just-mentioned ability to vary its depth of field, such a lens has certain disadvantages. Among the disadvantages are its cost and the increased susceptibility to mechanical failure over that of a fixed focus lens.
An electronic flash, fixed focus lens camera that is capable of forming sharp images of objects located closer to the lens than the near distance of its normal depth of field, has been disclosed. In Harvey U.S. Pat. No. 4,459,005 Harvey, for example, exposure control apparatus for an electronic flash, fixed focus lens camera includes means for controlling exposure with either of two predetermined apertures. The apparatus includes proximity sensing means for detecting when a subject to be photographed is within a given distance of the camera and a mechanism for restricting the exposure aperture to a size that is less than the aperture size the exposure control mechanism would normally employ when the subject is within the aforementioned given distance. The proximity sensing means includes a light emitting diode for illuminating a subject to be photographic with infrared (IR) light and a sensor for measuring subject IR reflectivity. A major disadvantage inherent in this type of electronic flash exposure control apparatus is that for some subject distances, subject reflectivities, and ambient scene light conditions, the selected aperture size may be small enough to produce a photograph with a sharp subject image but may be too small to produce an adequately exposed subject background. For other such conditions, the selected aperture size may be large enough to produce a photograph with an adequately exposed subject background but too large to produce a subject image of acceptable sharpness.
An electronic flash, fixed focus lens, scanning aperture camera that is capable of forming sharp images of subjects located closer to the camera than the near distance of the lens's normal depth of field has also been disclosed in U.S. patent application Ser. No. 07/414,267, filed Sep. 29, 1989 by Coltman et al., in common assignment wherewith. The camera disclosed therein is capable of automatically forming sharp images of such close subjects over a wider range and a greater number of subject distances than was theretofore possible. These close subject images are formed by firing the electronic flash at the smallest possible aperture for optimum image sharpness and overall scene exposure as a function of scene brightness and of subject IR reflectivity levels. While the exposure control system described in the Coltman et al. application is quite effective in forming sharp and well exposed images of such relatively close subjects and their associated scene backgrounds, the exposure control system requires a memory function for storing subject reflectivity or range related subject distance data obtained immediately prior to an exposure interval, for use during the exposure interval. This storage requirement necessarily increases exposure control system cost and complexity.